The article, "Guidance Characteristics of GNSS Landing Systems," Presented in the IEEE/AIAA Proceedings of the 17.sup.th Digital Avionics System Conference, November 1998, by D. Alexander Stratton, is herein incorporated by reference.
Precision-landing systems, including instrument landing systems (ILS), global navigation satellite system (GNSS) landing systems (GLS), and microwave landing systems (MLS), provide guidance to the pilot of an aircraft in the form of vertical and lateral deviations from an intended approach path known as the glide path or glide slope. In the aviation industry, technical and operational concepts for precision landing have evolved around the signal in space (SIS)characteristics of the ILS. Significant characteristics of ILS guidance arise from the SIS created by two antenna arrays positioned in the runway area. A glide-slope array located near the touchdown zone provides an ultra high frequency (UHF) signal for vertical guidance, while a localizer array typically located off the runway stop end provides a very high frequency (VHF) signal for lateral guidance.
Each signal contains 90 and 150 Hz amplitude modulations, and the antenna patterns are adjusted so that the modulation depths are equivalent along the final-approach path. Angular displacements from the final-approach path produce proportional changes in the depth of the individual modulations, so that a difference in depth of modulation (DDM) is sensed by an airborne ILS receiver. The proportionality between angular displacement and DDM, called displacement sensitivity, is maintained within specifications by adjustments of the antenna patterns. The receiver's DDM outputs can be displayed "raw" for manually piloted approaches and processed by flight-control systems to produce vertical and lateral steering commands for manual and automatically controlled landings.
In the avionics industry, GLS may be used to replace or supplement conventional ILS. The GNSS used in the GLS can be the global positioning system or other satellite positioning systems. Compatibility with conventional ILS would provide certain advantages to ease the process of replacement or supplementing. For GLS systems, the ILS characteristics described above are independent of the SIS. Instead, they are produced by algorithms inside the GLS receiver. The development of industry standards including the local-area augmentation system (LAAS) and the wide-area augmentation system (WAAS) raises numerous issues concerning the capabilities of these algorithms. Of concern are vertical guidance during glide-slope capture and flare, and lateral guidance during rollout. Other considerations include the impact on field measurement. Compatibility is of special value given the desire to efficiently introduce GLS technology into the anticipated multi-mode landing environment.